gnu: gdbm: Fix build with GCC 10.

* gnu/packages/patches/gdbm-gcc-compat.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/dbm.scm (gdbm)[source](patches): New field.

gnu/local.mk

@@ -1044,6 +1044,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gdb-hurd.patch				\
   %D%/packages/patches/gd-fix-tests-on-i686.patch		\
   %D%/packages/patches/gd-brect-bounds.patch			\
+  %D%/packages/patches/gdbm-gcc-compat.patch			\
   %D%/packages/patches/gdm-default-session.patch		\
   %D%/packages/patches/genimage-signedness.patch		\
   %D%/packages/patches/geoclue-config.patch			\

gnu/packages/dbm.scm

@@ -139,6 +139,7 @@ SQL, Key/Value, XML/XQuery or Java Object storage for their data model.")
               (method url-fetch)
               (uri (string-append "mirror://gnu/gdbm/gdbm-"
                                   version ".tar.gz"))
+              (patches (search-patches "gdbm-gcc-compat.patch"))
               (sha256
                (base32
                 "1p4ibds6z3ccy65lkmd6lm7js0kwifvl53r0fd759fjxgr917rl6"))))

gnu/packages/patches/gdbm-gcc-compat.patch

@@ -0,0 +1,16 @@
+Fix build failure with GCC 10 due to duplicate definitions.  It can be
+reproduced on older compilers with -fno-common.
+
+Fix taken from Gentoo: https://bugs.gentoo.org/705898
+
+--- a/src/parseopt.c
++++ b/src/parseopt.c
+@@ -255,8 +255,6 @@ print_option_descr (const char *descr, size_t lmargin, size_t rmargin)
+ }
+ 
+ char *parseopt_program_name;
+-char *parseopt_program_doc;
+-char *parseopt_program_args;
+ const char *program_bug_address = "<" PACKAGE_BUGREPORT ">";
+ void (*parseopt_help_hook) (FILE *stream);
+